BACKGROUND OF THE INVENTION The present invention relates to catalytic combustion, and provides a combustor suitable for use in a gas turbine. The catalytic combustor of the present invention not only can operate at high temperatures without deactivation of the catalyst, but also"lights off"at a relatively low temperature.

The present invention is an improvement over the catalytic combustors described in U. S. Patent Nos. 5,202,303,5,346,389, and 5,437,095, the dis- closures of which are hereby incorporated by reference. The combustors in the above-cited patents are formed from a strip of metal foil which is coated with a catalyst on only one side. The strip is folded back and forth upon itself to form catalyzed channels alternating with uncatalyzed channels. The strip is corrugated, with a herringbone pattern, so that when the strip is folded, the corrugations cross over each other and there- by maintain the spacing between the folds. The fuel-air mixture which flows through the catalyzed channels is combusted. The fuel-air mixture which flows through the uncatalyzed channels is not combusted, and cools the catalyzed surfaces.

In the operation of the combustor described above, a fuel-air mixture is first directed through the combustor, and the temperature of the mixture is gradually increased. Eventually, combustion will start in the catalyzed channels, at a temperature called the"light-off"temperature. For purpos- es of starting the combustion, a low light-off temperature is desirable. A low light-off temperature could be achieved by coating both sides of the strip, but the resulting high temperature would deactivate the catalyst.

Thus, there is a need for a catalytic combustor which lights off relatively easily, but which nevertheless can operate in high temperature environments without deactivation of the catalyst. The present invention provides such a combustor.

SUMMARY OF THE INVENTION The catalytic combustor of the present invention is formed of a strip of metal, preferably metal foil. One side of the foil is coated completely with a combustion catalyst. One edge of the other side of the foil is coated with catalyst in a narrow band extending along the length of the foil. The strip is then folded back and forth upon itself to form a cata- lytic combustor having a cross-sectional shape determined by the spacing of the folds along the length of the strip. The width of the foil strip becomes the length of the combustor, relative to the incoming gas flow.

The resulting combustor therefore includes channels having walls which are coated with catalyst, and channels having walls which are not coated, the coated channels alternating with the uncoated channels. In addition, the portions of the channels located at or near the inlet end of the resulting combustor are all coated with catalyst. The channels near the inlet end are cooled by radiating out into the open space upstream of the inlet. The band or region where the channels are fully coated should be no wider than about five times the nominal spacing between the folds of the strip. By coating all of the inlet portions of the channels with catalyst, one lowers the light-off temperature of the combustor. And by limiting the depth of the fully coated channels, one prevents damage to the combustor due to excessive temperature.

In an alternative embodiment, the combustor is formed as a spiral, by providing two strips, each of which is coated in the manner described above, arranging the strips such that the uncoated portion of one strip faces the uncoated portion of the other strip, and winding the pair of strips into a spiral. The resulting combustor again has alternating cata- lyzed and uncatalyzed channels, except near the inlet end, where all of the channels are catalyzed.

The present invention therefore has the primary object of providing a catalytic combustor.

The present invention has the further object of providing a catalytic combustor having a reduced light-off temperature.

The present invention has the further object of providing a catalytic combustor as described above, wherein the combustor is protected from ex- cessive temperature.

The present invention has the further object of providing a catalytic combustor as described above, wherein the catalyst is not deactivated dur- ing normal operation.

The present invention has the further object of providing a catalytic combustor that can be used in a gas turbine, or in other applications in- volving high temperatures.

The present invention has the further object of providing a catalytic combustor having a reduced light-off temperature, wherein the combustor can be formed into any of a plurality of possible shapes.

The present invention has the further object of providing a spiral catalytic combustor having a reduced light-off temperature.

The reader skilled in the art will recognize other objects and advant- ages of the present invention, from a reading of the following brief description of the drawings, the detailed description of the invention, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 provides a top view of a strip of metal foil, coated with catalyst, and used to form the catalytic combustor of the present inven- tion.

Figure 2 provides a bottom view of the strip of Figure 1, i. e. showing the opposite side of the strip, which side has been coated with catalyst only along a narrow band, according to the present invention.

Figure 3 provides a fragmentary cross-sectional view of a-combustor made according to the present invention, after the strip has been folded back and forth upon itself, indicating, by hatch marks, which portions are coated with catalyst and which portions are not coated.

Figures 4a and 4b provide schematic cross-sectional views of a spiral combustor made according to the present invention, wherein Figure 4a is a cross-section taken near the inlet end and Figure 4b is a cross-section taken further downstream of the inlet end.

DETAILED DESCRIPTION OF THE INVENTION The catalytic combustor of the present invention is made from a strip of metal, preferably corrugated metal foil. The strip is fully coated with catalyst only on one side. Figure 1 shows strip 10 coated with catalyst 12, the catalyst coating being indicated by cross-hatching in the drawing.

The dimensions of the strip are exaggerated in Figure 1 for clarity ; in practice, the ratio of the length to the width of the strip would be much greater than what is shown in the figure. The other side of strip 10 has a catalyst coating only on a narrow band 14, having a width b, the band being located adjacent to an edge of the strip and extending along. the entire length of the strip, as illustrated in Figure 2. The dimensions of the band are also not necessarily drawn to scale, but are exaggerated for pur- poses of illustration. The band 14 preferably has a substantially uniform width.

The corrugations in the strip are not shown in the drawings, for clarity of explanation. It is well known in the art to corrugate a strip or sheet of metal, before applying a catalyst coating, and before folding.

When the strip is folded back and forth upon itself to define the combus- tor, the corrugations establish a generally uniform spacing between ad- jacent folds. The locations at which the strip is folded determine the cross-sectional shape of the folded structure, as is also well-known in the art.

It will be appreciated that the width w of the strip, indicated in Figures 1 and 2, becomes the length L of the combustor (indicated in Figure 3), relative to the incoming gas flow. Also, the region corresponding to the narrow band of catalyst is positioned at the inlet end of the com- bustor, i. e. the end at which the incoming gas first enters the combustor.

Figure 3 provides a schematic cross-sectional view of a portion of the combustor, showing the areas that are coated with catalyst and those which are not coated. In the view of Figure 3, the folds are generally per- pendicular to the plane of the paper. Gas flows into the combustor in the direction indicated by arrow 15, and flows through the combustor towards its outlet end. The length L of the combustor, as seen by the incoming gas, corresponds exactly to width w shown in Figures 1 and 2. The diagram of Figure 3 is not drawn to the same scale as those of Figures 1 and 2. In particular, dimension b of Figure 2 is also represented in Figure 3, but not to the same scale.

In Figure 3, the catalyzed surfaces are indicated by hatch marks. The main body of the combustor (i. e. the portion which excludes the illustrated band having dimension b) includes catalyzed channels 21,23,25,27 alter- nating with uncatalyzed channels 22,24,26,28. However, in the region near the inlet end, to a depth represented by dimension b, all of the chan- nels are coated with catalyst. This full coating of the channels near the inlet exists because the original strip was fully coated on one side, and was also coated in a band of width b on the opposite side, as described above.

Thus, as shown in Figure 3, the combustor of the present invention comprises a structure which includes, in all but a small region near the inlet end, alternating catalyzed and uncatalyzed channels. In the region near the inlet end, all of the channels are catalyzed. One can therefore describe the combustor of the invention as one having a first plurality of channels alternating with a second plurality of channels, the first plural- ity of channels being fully catalyzed, and the second plurality of channels being catalyzed only in a region near the inlet end of the combustor.

The light-off temperature of the combustor formed of the above-de- scribed strip is lowered by coating the strip on both sides in the narrow band near the edge that becomes the inlet face of the combustor. However, the band has the disadvantage that it increases the temperature within the band. In the region in which the strip is coated on both sides, combustion will occur on both sides. If there is no cooling of the strip, and if the. doubly coated region is too deep, the temperature of the strip will reach the adiabatic combustion temperature of the fuel-air mixture. In a gas turbine, the latter temperature can be 1300° C, which would quickly deacti- vate the catalyst.

The portions of the combustor near the inlet end are cooled because they can radiate heat into the open space upstream of the inlet end. In general, the closer a point is to the inlet end, the more effectively that point will be cooled. The farther the point from the inlet end (i. e. the deeper it is located within the combustor), the less effective the cooling.

The latter concept is illustrated in Figure 3, which shows three points, labeled 1,2, and 4 which are inset from the inlet end of the combustor by 1,2, and 4 times the spacing between the strips. These points can radiate heat into the open space upstream of the inlet end, through angles a1, a2, and a4. The size of these angles clearly decreases as the points move into the interior of the combustor. Thus, the rate of cooling for point 4 will be less than that for point 2, and the rate of cooling for point 2 will be less than for point 1.

It is believed that if the band 14 of Figure 2 is wider than about five times the spacing of the folds of the strip, the temperature at the inlet end will approach the adiabatic combustion temperature, which is high enough to deactivate the catalyst, as explained in U. S. Patent No.

5,202,303. Thus, when coating the side of the strip illustrated in Figure 2, the narrow band should be less than about five times the nominal spacing of the folds.

The catalytic combustor of the present invention can alternatively be made in the form of a spiral. To make the spiral combustor, one begins with two identical strips, each strip being prepared exactly as shown in Figures 1 and 2. One arranges these strips such that the uncoated portion of the first strip faces the uncoated portion of the second strip. Then, the pair of strips is wound into a spiral. As before, the corrugations (not shown) in the strips keep adjacent layers apart. The resulting spiral combustor is illustrated by the two cross-sectional views of Figures 4a and 4b. For simplicity of illustration, only a few turns of the spiral are shown, the continuation of the spiral being represented by the broken lines.

The cross-section of Figure 4a is taken along a plane which is no farther than the distance b (see Fig. 2) from the inlet end of the spiral combustor. At the inlet end, both sides of each strip are coated, due to the presence of catalyst band 14. Therefore, Figure 4a shows a spiral com- bustor in which all channels are fully catalyzed.

The cross-section of Figure 4b is taken along a plane which is posi- tioned at a distance greater than b from the inlet end. At this position, each strip is coated on only one side, because the cross-section is taken along a plane which intersects the uncoated portion of every strip. Thus, Figure 4b shows a spiral combustor in which every other channel is cata- lyzed. In short, Figures 4a and 4b together represent a spiral combustor in which all channels are catalyzed within a distance b from the inlet end, and every other channel is catalyzed beyond a distance b from the inlet end.

The spiral combustor illustrated in Figures 4a and 4b works in exactly the same way as the embodiment described earlier.

Instead of starting with two separate strips and winding the pair into a spiral, it is also possible to make the spiral from a single strip. In the latter alternative, one folds the strip at its midpoint, so as to create a structure having two layers, and then one winds the two-layered structure into a spiral. The result is, for practical purposes, the same product as the spiral combustor described previously, and should be consid- ered an equivalent thereof.

The catalytic combustor of the present invention therefore provides the advantages enjoyed by the above-cited patents, due to its alternating catalyzed and uncatalyzed channels, while also having a reduced light-off temperature, due to the coating of both sides of the strip in the positions corresponding to the band shown in Figure 2. The combustor of the present invention can therefore be used in high temperature environments without destroying the activity of the catalyst due to excessive temperature.

The invention can be modified in various ways. The dimensions shown in the figures are illustrative and not limiting. As long as the dimension b is not greater than about five times the nominal spacing of the folds of the strip, the value of that dimension can be varied considerably. The combustor can be formed in a wide variety of shapes, determined by the locations at which the strip is folded. The invention is not limited by the type of catalyst, or the means by which the catalyst is attached to the combustor. These variations, and others which will be apparent to those skilled in the art, should be considered within the scope of the present invention.